Hydrocephalus syndrome still represents a complex, difficult challenge, as regards not only understanding of the pathogenetic mechanisms underlying it, but also the identification and implementation of the best possible treatment method.
Numerous pathogenetic hypotheses of hydrocephalus have been proposed in the last few decades.
For the sake of completeness, the hypotheses which have been and are currently most widely accredited will be described briefly below.
The Cerebrospinal Fluid “Circulation” Hypothesis
The first hypothesis, which is also the most widely accepted, is based on the assumption that cerebrospinal fluid (CSF) “circulates” from the choroidal plexuses of the cerebral ventricles, its final destination being the arachnoid granulations of the cerebral convexity.
According to this theory, an obstacle to this “circulation” at any level and by any means, caused, for example, by stenosis of the aqueduct, a tumour of the posterior cranial fossa, or reduced absorption following haemorrhagic or infectious events affecting the subarachnoid spaces, gives rise to “obstructive” or “non-communicating” hydrocephalus as well as “non-obstructive” or “communicating” hydrocephalus.
Leaving aside the theoretical arguments and the extensive set of indirect, clinical and experimental data contradicting the pathogenetic hypothesis of an obstacle to the outflow of CSF, what finally demolished this first hypothesis was “direct” evidence, in human and animal models, of the absence of “circulation” of cerebrospinal fluid, at least in the form it would need to take in order for a mechanism involving an “obstacle” to that circulation to produce significant ventricular dilation.
In other words, it has been extensively demonstrated firstly that there is a “diffusion”, not a movement of volume (ie. “circulation”) of CSF in the intracranial system, so that the sites where CSF is produced are also responsible for its absorption and vice versa, and secondly that the movement of CSF inside and outside the ventricular cavities is constituted by a periodic oscillation synchronous with the cardiac pulsation, but without a real “net flow” in either direction.
This last finding is based on extensive medical literature on the subject, bearing in mind that the fundamental variable to be considered is always the rate of flow, namely the “void signal”.
In fact, the data obtained by experimental measurement must be suitably corrected to take account of the variation in the gauge of the structure in which the measurement is taken (usually the mesencephalic duct), which periodically narrows during systole, thus causing an increase in the speed (but not the flow), and widens during diastole, causing the opposite effect which exactly offsets the previous one, thus confirming the absence of a “net flow”, and consequently of true “circulation” of CSF.
The “Ventricular Pulse Pressure” Hypothesis
The second hypothesis is based on the observation that hydrocephalus, whether clinical or experimental, is very often associated with an increase in the “CSF pulse pressure”, namely the difference between the maximum and minimum values of the intracranial pressure during each cardiac cycle.
If this second hypothesis is analysed on the basis of an intracranial system model with a rigidly constant volume, a different explanation can be formulated for the onset of hydrocephalus syndrome, namely that the development of hydrocephalus of any type, regardless of its etiology, is produced by the association between the “intraventricular pulse pressure of the choroid plexuses” and an “asymmetrical response” by the cerebral parenchyma.
In particular, this second hypothesis takes account of a behavioural characteristic well known in the literature in relation to viscoelastic substances, with which the cerebral parenchyma has always been compared in anatomical and structural terms.
This means that the brain, as a result of the pulsating force acting on it, is more easily “compressible” (during systole) then “expandable” (during diastole) at the end of the compression phase.
The continual alternation between systole (compression) and diastole (expansion) leads to a progressive reduction in cerebral volume and consequently an increase in ventricular volume, until a new balance is reached between the forces acting in the two directions that will determine the actual dimensions of the cerebral ventricles, from normal or relatively normal volumes to the extreme degrees of hydrocephalus syndrome.
The factors illustrated above explain why the current systems used to treat hydrocephalus, mainly based on the use of “CSF shunts” which move part of the CSF volume from the intracranial system to other body cavities, can provide no more than a partial, indirect, imprecise solution to the pathogenetic alteration postulated here.
This incongruity clearly explains the limitations still involved in the conventional treatment of hydrocephalus syndrome.
A radical solution to the problem only seems to be possible if one or both of the two factors illustrated above can be modified, namely the “intraventricular pulse pressure” and the “asymmetrical response” of the cerebral parenchyma.